In general, inkjet printing machines or printers include at least one printhead that ejects drops or jets of liquid ink onto a recording or image forming surface. An aqueous inkjet printer employs water-based or solvent-based inks in which pigments or other colorants are suspended or in solution. Once the aqueous ink is ejected onto an image receiving surface by a printhead, the water or solvent is evaporated to stabilize the ink image on the image receiving surface. When aqueous ink is ejected directly onto media, the aqueous ink tends to soak into the media when it is porous, such as paper, and change the physical properties of the media. Because the spread of the ink droplets striking the media is a function of the media surface properties and porosity, the print quality can be inconsistent. To address this issue, indirect printers have been developed that eject ink onto a blanket mounted to a drum or endless belt. The ink is dried on the blanket and then transferred to media. Such a printer avoids the changes in image quality, drop spread, and media properties that occur in response to media contact with the water or solvents in aqueous ink. Indirect printers also reduce the effect of variations in other media properties that arise from the use of widely disparate types of paper and films used to hold the final ink images.
In aqueous ink indirect printing, an aqueous ink is ejected onto an intermediate imaging surface, typically called a blanket, and the ink is partially dried on the blanket prior to transfixing the image to a media substrate, such as a sheet of paper. To ensure excellent print quality, the ink drops on the blanket must spread and not coalesce prior to drying. Otherwise, the ink images appear grainy and have deletions. The lack of spreading can also cause missing or failed inkjets in the printheads to produce streaks in the ink image. Spreading of aqueous ink is facilitated by materials having a high energy surface. In order to facilitate transfer of the ink image from the blanket to the media substrate, however, a blanket having a surface with a relatively low surface energy is preferred. These diametrically opposed and competing properties for a blanket surface make selections of materials for blankets difficult. Reducing ink drop surface tension helps, but the spread is still generally inadequate for appropriate image quality. Offline oxygen plasma treatments of blanket materials that increase the surface energy of the blanket have been tried and shown to be effective. The benefit of such offline treatment may be short lived due to surface contamination, wear, and aging over time.
One challenge confronting indirect aqueous inkjet printing processes relates to the spread of ink drops during the printing process. Indirect image receiving members are formed from low surface energy materials that promote the transfer of ink from the surface of the indirect image receiving member to the print medium that receives the final printed image. Low surface energy materials, however, also tend to promote the “beading” of individual ink drops on the image receiving surface. Since a printer partially dries the aqueous ink drops prior to transferring the ink drops to the print medium, the aqueous ink does not have an opportunity to spread during the printing process. The resulting printed image may appear to be grainy and solid lines or solid printed regions are reproduced as a series of dots instead of continuous features in the final printed image. To address these issues, a surface maintenance unit in an aqueous inkjet printer applies a layer of a hydrophilic composition comprising a liquid carrier and an absorption agent to the image receiving surface. A dryer is positioned and configured to remove at least a portion of the liquid carrier from the layer of hydrophilic composition after the surface maintenance unit has applied the hydrophilic composition to the image receiving surface to form a dried layer of the absorption agent. After a plurality of inkjets ejects aqueous ink onto the dried layer to form an aqueous ink image on the image receiving surface, a transfix member engages the image receiving member to form a transfix nip and apply a pressure to a print medium moving through the transfix nip to transfix the aqueous ink image and at least a portion of the dried layer to a surface of the print medium.
This aqueous inkjet printer generally works well; however, some print jobs present issues that impact the transfixing of the ink image to the media in the nip. Specifically, regulation of the dryers and heaters in printers configured as described above evaporate water from the hydrophilic composition and ink with reference to a density of the ink on the blanket. Issues can arise when the ink image on the blanket has varying densities of ink. For example, some images have areas that are relatively solid, that is, each pixel in the area has colorant in it, while other areas are halftone, that is, some percentage, such as fifty percent, of the pixels in the area have colorant and the remaining pixels are empty of ink. If the dryers and heaters are controlled to ensure the solid areas are appropriately dried, then the halftone areas may be completely dried. Consequently, the solid areas of the image are likely to transfer well, but the halftone areas only partially transfer, if at all. The resulting dropout of colorant in the image adversely impacts the overall image quality. Being able to preserve the advantages of the hydrophilic composition and enabling all areas of an ink image to transfer to the media regardless of the ink density would be beneficial.